Cow fertility – not so black and white

Increased milk yields in Holstein cows have unfortunately led to lower fertility. But finding new genetic markers at early reproductive stages may help boost fertility

Holstein cattle - the black-and-white dairy cows you might see in a child’s picture book - have been bred in northern Europe for hundreds of years. Over the last few decades, better management and selective breeding of the most productive cows have led to steady increase in milk production per cow. But more milk has had a downside; namely, a decline in fertility. Now, the FECUND project, started in 2013, has begun to investigate Holstein biology to reverse the fall in fertility. The project hopes to intervene by identifying genetic ways of spotting cows that can support the early stages of reproduction. This will then allow breeders select for more fertile animals.

In particular, the project scientists hope to find new genetic markers connected with fertility. Specifically at early reproductive stages, which is the time when most Holstein embryos are lost. “We expect to find new genetic markers that could allow us better discriminate animals with a high capacity for fertility from those with less. This is very important for breeding,” says project geneticist, Eckhard Wolf of the Gene Center at the Ludwig Maximillians University, in Munich, Germany.

Milk yields in Holstein cows have steadily increased in recent decades. For instance, milk per cow rose from 5,000 to 7,500 litres between 1980 and 2012, in the UK, according to estimates. However, a focus on only selecting for milk quantity meant that other important traits, like fertility, were ignored. There have also been mistakes in breeding. “Major mistakes were inadvertently made in the 1960s and 1970s. Elite bull mothers with extremely high milk yields were used to produce bulls,” explains dairy cattle expert, Phil Garnsworthy, professor of dairy science at the University of Nottingham, UK. “Their low fertility was masked by hormone treatment regimens. Genes for high milk yield and low fertility were passed on to bulls that were widely used throughout the population,” he tells CommNet. The result has been great milk yields but falling fertility in the Holstein breed.

Although breeding caused this problem, it can fix it too. Wolf predicts that breeders and ultimately farmers will benefit from the project. “We have already identified genes that are expressed in oocytes [eggs] and the next step is to look at the genomes of breeding animals and see if we can find variants of these genes that are associated negatively or positively with fertility,” he adds.

However, significant challenges need to be overcome. Garnsworthy explains that fertility, unlike milk yield, which is a strongly inherited trait, is more complicated. Although genes are important, other factors such as management, the environment and nutrition are key too. “The project focusses on one aspect of the fertility problem: early embryo loss,” he tells CommNet. Project scientists note that selection on general reproductive performance is limited because of low heritability and accuracy. Physiological traits have higher heritability and are more likely to be linked to genetic markers. “The objective of finding some of these markers has a high chance of success,” Garnsworthy tells CommNet. He warns, however, that “caution will be needed to ensure that they do not select against good ovulation and fertilisation rates.”

But there is also optimism that the project could bring practical benefits by reversing internationally declining cow fertility levels. Gary Rogers, a US expert in cattle breeding and advisor to breeding organisation Geno Global says the project’s objectives “are achievable” and “would be very helpful to breeders and dairy industries around the world.”

“Heavy selection for production traits and for thinner cows has been a mistake without the simultaneous selection for improved production. In addition higher inbreeding in most populations has contributed to the decline,” Rogers notes. He concludes: “Reproductive performance is a major driver of efficient dairy production. It also has a huge impact on herd profitability so it is important for herd financial success.”